Mice injected from birth with goat antiserum to mouse micron chain (micron-suppressed) were utilized to examine the requirement for immunocompetent B-cells in the immune response to 17NL P.b. yoelii malaria (ordinarily non lethal), and to X-irradiated P.b. berghei sporozoites. Previous work showed that T cells were required for resistance of BALB/c mice to 17XNL P.b. yoelii. Present experiments demonstrated increased parasitemia and mortality of micron-suppressed BALB/c mice compared to controls, implicating an important role for B cells. In contrast, micron-suppressed BLCF1 mice immunized with sporozoites resisted a challenge infection in the majority of cases. Adult thymectomized, X-irradiated, bone marrow reconstituted, and antithymocyte serum treated BLCF1 mice were not effectively immunized using the same protocol, suggesting an essential role for T cell immunization in sporozoite immunity. We have studied a variety of parameters during the course of infection of BALB/c mice with 17XNL P.b. yoelii including parasitemia, hematocrit, antimalarial indirect hemagglutinating antibody and in vitro responses of spleen cells to malarial antigens (T dependent) and mitogens. A dynamic series of events was observed, characterized by rapid T cell sensitization and a period of depressed in vitro spleen cell responsiveness during the period of peak parasitemia.